What does viscosity control on a flexo press? Water-based and solvent inks are delivered to the plate through an anilox roll, and viscosity decides how much ink the cells release and how quickly it flows back level. Thin ink releases readily and fills in; thick ink sits on the surface and dries before it transfers. The press operator adjusts viscosity to hold a target, and that target is one of the few variables that can change a job within a single shift.
How is viscosity measured? A flow cup is the standard shop-floor tool: the time for a fixed volume of ink to drain through a calibrated orifice, reported in seconds with the cup number stated, because a reading without the cup size means nothing. Automatic systems use a inline viscometer and dose solvent to hold the set point, which removes the drift that manual checks allow between measurements. The target is set per ink and per job, not per press, and it has to agree with the colour target the job was approved against.
What happens when ink is too thin? The anilox releases more ink than intended, so dots spread, fine type fills in and solids become mottled as the ink floods and dries unevenly. Colour strength drops, so the operator compensates with density and the print looks flat rather than weak. On corrugated, thin ink wicks into the liner and the edge definition of every letter softens, which is visible on a shipping case at arm’s length.
What happens when ink is too thick? Transfer becomes uneven, the plate picks up dried ink and the print shows dirty patches, and the ink can bridge the cells of the anilox so the laydown varies across the web. Drying suffers because a thick film skins over while the layer beneath stays wet, which shows up later as blocking in the stack. Foaming is another symptom, particularly on a fast press with a small ink pan.
Why does pH matter as much as viscosity? Water-based inks are formulated to sit in a mildly alkaline range, and the resin that carries the pigment depends on that range to stay dissolved. As the ink runs, pH drifts down, viscosity rises and the ink begins to dry on the plate. Operators who add water to thin a drifting ink can fix the reading for a few minutes and make the underlying problem worse, because water lowers both viscosity and pH.
How often should it be checked? At the start of each run, after any ink addition, and at intervals through the shift, with readings logged rather than remembered. A job that starts at the target and ends three hours later at double the reading will show a colour shift from the first sheet to the last, and the cause is usually evaporation rather than the ink supplier. Automatic control hides the drift but does not remove the need to record it.
What should a buyer ask? Whether viscosity and pH are controlled and logged on the job, what the target readings are for the inks used, and whether the press has automatic dosing. Those questions are unusual enough to be revealing: a printer who answers them precisely is measuring the run, and a printer who does not is setting colour by eye and hoping the shift holds together.
Chengtop prints corrugated cases and cartons on flexo and offset lines in Wuhan, with ink viscosity logged through each run. Send the artwork and the substrate, and we will confirm how the ink is controlled on the process we propose.
